1. Field of the Invention
This invention relates to mixtures of methyltin trichloride and dimethyltin dichloride in certain proportions, and to a convenient, industrially practical process for the manufacture of such mixtures of satisfactory quality to be used without purification to make industrially useful mixtures of methyltin and dimethyltin compounds, for example methyltin trimercaptide and dimethyltin dimercaptide stabilizers for vinyl chloride polymers. Descriptive references to these mercaptide stabilizers for halogen-containing polymers are abundantly cited in U.S. Pat. No. 4,134,868, to Minagawa et al, issued on Jan. 16, 1979, and incorporated herein by reference.
2. Prior Art
Dimethyltin dichloride can be prepared by a redistribution reaction of methyltin compound having more than two methyl groups linked to a tin atom, i.e. tetramethyltin and/or trimethyltin chloride, with a tin compound having fewer than two methyl groups linked to a tin atom, i.e. methyltin trichloride and/or stannic chloride, as disclosed, for example by D. Grant et al in Journal of Organometallic Chemistry 1965, vol. 4, pages 229-236, and by E. Van den Berghe et al, ibid. 1966, vol. 6, pages 522-527. According to these references, the following reactions, not requiring the use of a catalyst, lead to a dimethyltin dichloride: EQU (CH.sub.3).sub.3 SnCl+CH.sub.3 SnCl.sub.3 .fwdarw.2(CH.sub.3).sub.2 SnCl.sub.2 EQU (CH.sub.3).sub.4 Sn+CH.sub.3 SnCl.sub.3 .fwdarw.(CH.sub.3).sub.3 SnCl+(CH.sub.3).sub.2 SnCl.sub.2 EQU (CH.sub.3).sub.3 SnCl+SnCl.sub.4 .fwdarw.(CH.sub.3).sub.2 SnCl.sub.2 +CH.sub.3 SnCl.sub.3
The consumption of tetramethyltin in these reactions is rapidly completed, but trimethyltin chloride is incompletely consumed when the reactions reach equilibrium and no further conversion takes place. The use of the highly toxic tetramethyltin or trimethyltin starting material and unreacted trimethyltin chloride in the finished product is a serious drawback of this procedure. Moreover, the trimethyltin mercaptides formed from mixtures containing trimethyltin chloride are also toxic and can only be tolerated as stabilizers for vinyl chloride polymers at carefully controlled low levels well below 1% by weight of the stabilizer, as disclosed for example by L. Weisfeld et al in U.S. Pat. No. 3,887,519 of June 3, 1975.
As is seen in the above reaction equations, methyltin trichloride can be both formed and consumed in this redistribution reaction. This reaction, however, is not a practical method for manufacturing methyltin trichloride, as is pointed out in the above mentioned Van den Berghe et al article, and it is limited to tetramethyltin and trimethyltin compound starting materials. For example, the disclosure in Weisfeld et al, supra, includes an example of dimethyltin dichloride purification to remove a trimethyltin chloride contaminant by using dimethyltin dichloride containing 5% trimethyltin chloride with 2.6% by weight of stannic chloride for 2 hours at 120.degree. C. The trimethyltin chloride being thus reduced to 0.5% in the dimethyltin dichloride. Weisfeld et al states that this purification treatment follows the reaction equations: EQU (CH.sub.3).sub.3 SnCl+SnCl.sub.4 .fwdarw.(CH.sub.3).sub.2 SnCl.sub.2 +CH.sub.3 SnCl.sub.3, (a)
and EQU 2(CH.sub.3).sub.3 SnCl+SnCl.sub.4 .fwdarw.3(CH.sub.3).sub.2 SnCl.sub.2 (b)
Calculation shows that a 2.6% SnCl.sub.4 addition to a 5.0% (CH.sub.3).sub.3 SnCl product can only lower the (CH.sub.3).sub.3 SnCl content to 1% or less if reaction (b) goes to completion to the exclusion of reaction (a). Accordingly, substantially no methyltin trichloride is formed according to Weisfeld's disclosure.
Langer in U.S. Pat. No. 3,454,610 of July 8, 1969 disclosed the possibility of extending the stannic halide plus alkyltin compound redistribution reaction to the use of disubstituted organometallic halide such as dimethyltin dichloride with stannic chloride, by using as a reaction medium an aliphatic sulfoxide which results in the in situ formation of a sulfoxide complex of the redistribution product. Thus a reaction of 0.05 mole dimethyltin dichloride and 0.05 mole stannic chloride with 0.7 mole dimethyl sulfoxide gave, after precipitation in 1:1 benzene-alcohol, a 90% yield of a white crystalline complex of methyltin trichloride with two moles per mole of methyltin trichloride of dimethyl sulfoxide.
An additional operation of vacuum stripping near room temperature is required, according to Langer, to eliminate dimethyl sulfoxide from the recovered complex and isolate the desired methyltin trichloride.
Neumann in U.S. Pat. No. 3,459,779 of Aug. 5, 1969 disclosed that alkyltin trihalides are obtained from dialkyltin dihalides and tin tetrahalide in the presence of polar substances, particularly phosphorus oxyhalide mixed with phosphoric acid. Isolation of the alkyltin trihalide made by Neumann's process requires fractional distillation to separate the product from excess stannic halide and phosphorus oxyhalide as well as from non-volatile phosphoric acid.
Kugele et al in U.S. Pat. No. 3,862,198 of Jan. 21, 1975 disclosed the preparation of alkyltin trihalide from stannic halide and an alkyltin compound having 2, 3, or 4 alkyl groups in the presence of an onium salt catalyst. As onium compounds they disclose compounds of the formula R.sub.4 ZY where R is alkyl, aryl, or aralkyl, Z is N, P, or As, and Y is an anion such as halide, sulfate, phosphate, nitrate, acetate, or trihalostannite. Kugele et al warn of a possible decomposition of the desired alkyltin trihalide into alkylhalide and stannous halide, and suggest adding alkyl halide, for example methyl chloride, to suppress this side reaction. There is no disclosure by Kugele et al how to isolate the desired alkyltin trihalide product from the onium salt catalyst. The presence of catalyst in the product contributes to its instability during storage and decreases the effectiveness of the methyltin mercaptide stabilizer prepared therefrom.
A process for the preparation of mixtures of dimethyltin dichloride and monomethyltin trichloride from the reaction of dimethyltin dichloride with stannic chloride, without catalyst, is now developed wherein the disadvantages of the foregoing processes are overcome. By the present process an intermediate mixture is produced without the formation of unwanted methyl chloride, and said mixture is reacted, without further treatment, with a mercaptan, as known in the art, to produce the corresponding vinyl chloride polymer stabilizers.